<title>STRUCT keyword </title>

<head>
<script language="JavaScript">
</script>
</head>

<body bgcolor="#ffffcc">
<hr>
<center><h1>STRUCT keyword </h1></center>
<hr>
<p>

<ul>
<li><a href="#basics">Structure basics.</a>
<li><a href="#membership">Structure membership.</a>
<li><a href="#pointers">Pointers to structures.</a>
<li><a href="#array">Array of structures.</a>
<li><a href="#geek">The bottom draw..</a>
<li><a href="../../CPLUSPLUS/SYNTAX/struct.html">C++ extensions</a>
</ul>

<a name=basics>
<hr>
<h2>Structure basics</h2>

<b>struct</b> is used to declare a new 
<a href="../CONCEPT/data_types.html">data-type</a>. 
Basically this means
grouping variables together. For example, a struct data type could be used 
to declare the format of the following file.<p>

<center>
<table border=1 bgcolor="ivory">
<tr><td>
<pre>          
   Jo		Loss		Maths		A   
   Harry	Carpenter	English 	A
   Billy	King		Maths		C
</pre>
</td></tr>
</table>
</center>
<p>

The records above could be described in a struct as follows:
<p>
<center>
<table border=1 bgcolor="ivory">
<tr><td>
<pre>   

   struct
   {
       char cname[8];
       char sname[16];   
       char exam[16];
       char grade;
   } record;

</pre>
</td></tr>
</table>
</center>
<p>
The statement above declares a variable called <b>record</b> with 4 members
called <b>cname, sname, exam, grade</b>. The structure as a whole can be 
referred to as <b>record</b> and a member can be referenced as 
<b>record.exam</b>
<p>

Structures can be declared in various forms...
<p>

<pre>
	struct x {int a; int b; int c;};		/* declaration	*/
	struct 	 {int a; int b; int c;} z;
	struct x z;
</pre>
All the examples above are structure declarations, 
<ul>
<li>The first gives <b>x</b> as a 'structure tag' - this is optional.
<li>The first and second declare the members of the structure.
<li>Second and third give <b>z</b> this is the variable that assumes the 
structure type.
</ul>
<p>

<a name=membership>
<hr>
<h2>Structure membership</h2>

We can access individual members of a structure with the . operator.<p>
For example to assign a value, we can enter:
<p>
<center>
<table border=1 bgcolor="ivory">
<tr><td>
<pre>
	
   struct x {int a; int b; int c;};   

   main()
   {
      struct x z;
     
      z.a = 10;
      z.b = 20;
      z.c = 30;
   }
   
</pre>
</td></tr>
</table>
</center>
<p>
And to retrieve a value from a structure member:
<p>
<center>
<table border=1 bgcolor="ivory">
<tr><td>
<pre>

   struct x
   {
      int a;
      int b;
      int c;
   };

   main()
   {
      struct x z;
	
      z.a = 10;
      z.a++;

      printf(" first member is %d \n", z.a);   
   }
   
</pre>
</td></tr>
</table>
</center>
<p>
<a name=pointers>
<hr>
<h2>Pointers to structures</h2>

<a href="#arrow">Fast path to an explanation of the -> operator.</a>
<p>

All that we have discussed so far has been OK but runs into problems 
when structures have to be moved between functions for the following 
reasons.
<ul>
<li>If the structure is large it is more effiecent to pass a 
<a href="../CONCEPT/pointers.html">pointer</a> to the structure
instead of the structure its self. This technic is also used to pass
<a href="../CONCEPT/pointers.html#arrays">pointers to arrays</a> between 
<a href="functions.html#2.5">functions.</a>
<p>
<li>When passing a structure to a function, you actually pass a COPY of the 
structure. Therefore it is not possible to change the values of members
within the structure as the copy is destroyed when the function ends.
</ul>
<p>
So how does it all work? Here is an example. (make your browser W-I-D-E so
you can see the two examples).
<p>
<center>
<table border=1 bgcolor="ivory">
<tr><td>
<pre>
					|
					|
   struct x {int a; int b; int c;} ;	| struct x {int a; int b; int c;} ;
					|
   void function(struct x);		| void function(struct x *);
					|
   main()			       	| main()
   {					| {
     struct x z;                        |   struct x z, *pz;	     <font color=red>/* 3 */</font>
					|   pz = &z;		     <font color=red>/* 4 */</font>
     z.a = 10;		<font color=red>/* 1 */</font>		|   z.a = 10;
     z.a++;				|   z.a++;
					|
     function(z);	<font color=red>/* 2 */</font>	        | function(pz);		     <font color=red>/* 5 */</font>
   }					| }
					|
   void function( struct x z)		| void function(struct x * pz)
   {					| {			     <font color=red>/* 6 */</font>
     printf(" first member %d \n", z.a);|   printf(" first member %d \n", (*pz).a);   
   }					| }
					|
</pre>
</td></tr>
</table>
</center>
<p>
Here is the annotation.
<ol>
<li>Give a structure member a value.
<li>Pass a COPY of the whole structure to the function.
<li>Define 'pz' a pointer to a structure of type 'x'.
<li>Put the address of 'z' into 'pz'. 'pz' now POINTS to 'z'.
PLEASE NOTE. 'z' is defined to reserve memory equal to the size of the 
structure. 'pz' only holds an address so will be 4 bytes long.
<li>Pass the pointer into the function.
<li>Print the value of the member 'a'.
</ol>

<a name=arrow>
The <b>(*pz).a</b> syntax is used a great deal in C and it was decided to create
a short hand for it. So:

<pre>
	(*pz).a   ==  pz->a
</pre> 

Here is the final picture.
<pre>

   /*************************************************************************/

   struct x {int a; int b; int c;} ;	/* Declare the structure.	    */

   void function(struct x * );		/* Declare the function.	    */

   /*************************************************************************/

   main() 
   {  
					/* Declare two variables.
					 * z == type struct x
					 * pz == a pointer to type struct x
					 */
     struct x z, *pz; 			

     pz = &z;        			/* put the address of 'z' into 'pz' */
     z.a = 10;				/* initialize z.a		    */
     z.a++;				/* Increment z.a		    */

					/* print the contents of 'z.a'
					 * using the pointer 'pz'	    */
   
     printf(" first member before the function call %d \n", pz->a);

					/* Call 'function' passing the 
					 * pointer 'pz'			    */
     function(pz);  			

					/* Print the NEW value of 'z.a'
					 * using three different notations  */
     printf(" first member after the function call %d \n", pz->a);
     printf(" first member after the function call %d \n", (*pz).a);
     printf(" first member after the function call %d \n", z.a);

   }

   /*************************************************************************/
   
   void function(struct x * pz)
   { 
					/* Print the value of 'z.a' by
					 * referencing the pointer 'pz'
					 * which holds the address of 'z'   */
     printf(" first member inside the function %d \n", pz->a);
      
					/* Increment the value of 'z.a'
					 * this is the source location
					 * in memory.			    */
     pz->a++;
     
   }

   /*************************************************************************/
  

</pre>

<a name=geek>
<p>
<hr>
<h2>The Bottom Draw</h2>

Finally, here is a little feature that allows you to save a little
space.
<p>
<center>
<table border=1 bgcolor="ivory">
<tr><td>
<pre>

  main()
  {
    struct Flags
    {
      unsigned int Online  <font color=red>:1</font>;   
      unsigned int Mounted <font color=red>:1</font>;
    }

    struct Flags TapeInfo;

    TapeInfo.Online  = 1;
    TapeInfo.Mounted = 0;
  }
  
</pre>
</td></tr>
</table>
</center>
<p>
The <font color=red>:1</font> tells the compiler that only 1 byte is
required for <b>Online</b> and <b>Mounted</b>. There are a few points to
note about this though.
<ul>
<li>You may expect the compiler to reserve 2 bytes for the structure, it
actually reserves one word (usually 4 bytes) as this is the smallest unit
that can be reserved, the remaining 2 bytes are unavailable.
This is still better than the 2 words that wold normally get reserved.
<li>You can put any number into the variable, if the number is too large
to fit, the high order bits are lost without warning.
<li>Only <b>signed int</b>, <b>unsigned int</b>, <b>int</b> support this syntax.
</ul>
<hr>

<h2>Examples</h2>
<p>
<img src="../../GRAPHICS/computer.gif">
<a href="../EXAMPLES/struct1.c">
This is the most basic <b>struct</b> example I could think of.</a>
<br>

<img src="../../GRAPHICS/computer.gif">
<a href="../EXAMPLES/struct2.c">
Using structure elements, and passing them into a function.</a>
<br>

<img src="../../GRAPHICS/computer.gif">
<a href="../EXAMPLES/struct3.c">
Passing a whole structure to a function.</a> This performs a copy of the 
structure so the same rules apply as for <b>int</b> etc. 
Pointers to structures can be passed but I have not got to them yet....
<br>

<a name=array>
<img src="../../GRAPHICS/computer.gif">
<a href="../EXAMPLES/struct4.c">Define and use an array of structures.</a>
<br>

<img src=../../GRAPHICS/help.gif>
<a href="../PROBLEMS/problems.html#struct">
Here is a <b>struct</b> problem for you.</a>



<hr>
<h2>See Also:</h2>
<img src=../../GRAPHICS/whiteball.gif>
<a href="typedef.html">typedef</a> keyword.
<br>
<img src=../../GRAPHICS/whiteball.gif>
<a href="../MISC/linklists.html">Linked lists</a>.
<br>


<p>

<hr>
<p>
<center>
<table border=2 width="80%" bgcolor="ivory">
<tr align=center>
<td width="25%">
<a href="../cref.html">                  Top</a>
</td><td width="25%">
<a href="../master_index.html">          Master Index</a>
</td><td width="25%">
<a href="keywords.html">                 Keywords</a>
</td><td width="25%">
<a href="../FUNCTIONS/funcref.htm">      Functions</a>
</td>
</tr>
</table>
</center>
<p>
<hr>
<address><a href="../../address.html">Martin Leslie</a> 
</address><p>
</body>
</html>
